In gaseous discharge devices adapted for use as display or storage apparatus, arrays of parallel conductors oriented at transverse angles to each other are disposed on opposite sides of a gas filled panel, the conductors being insulated from direct contact with the gas by a layer of dielectric. One example of such a gaseous display is described in U.S. Pat. No. 3,559,190, "Gaseous Display and Memory Apparatus", issued Jan. 26, 1971, to Donald L. Bitzer et al and assigned to the University of Illinois. Individual discharge sites located at coordinate intersections on said panel are selectively fired by application of high voltage drive signals composed of write pulses algebraically added to alternating sustain signals. When thus fired, the resulting light emitted from the selected sides forms one element or dot of the desired display, and a plurality of fired cells in a specified configuration forms a display. Signals to control the selection of the individual cells to form a display are low voltage digital signals which may originate from a computer, teletype unit, telephone line, local or remote display control unit, etc., while the combined sustain and write signals required to fire or discharge a cell may extend 300 volts in amplitude. To fire a cell, as more fully described hereinafter, a write pulse signal in the order of 50 volts amplitude is algebraically added to the sustain signal in the order of 300 volts peak-to-peak to obtain a signal which exceeds the firing potential of the gas, since the sustaining voltage of itself is insufficient to initiate a discharge. The panel logic circuits and panel control signal power supplies are electrically floated on the sustain signal using the sustain signal as a reference. By referencing both the pulsing circuits and logic circuits to the sustain potential, low voltage circuits adapted for integrated circuit packaging may be used for pulsing and logic functions, thereby limiting high voltage signals to the sustain signal source. By utilizing integrated circuitry for the logic and control pulsing functions, the cost and size of these circuits are significantly reduced. By utilizing a pulsed supply for the write and erase functions, rather than a fixed power supply, power is utilized only when pulsing, thus providing an additional saving in power consumption with its concommitant heat dissipation. Isolation between the digital control signals and the high voltage operating signals is provided by transformer coupling, and the panel selection logic is adapted to provide group and sub-group selection, as for example, individual line selection for the respective rows of the display. The present invention provides a system for interfacing various signal levels, and by providing a common reference permits communication between a processor or control unit, logic and selection circuitry and high voltage panel driving circuitry.
In addition to the reference feature wherein the interfacing logic circuits and pulsing circuits are electrically floated on the sustain voltage for the panel, the present invention provides a low-cost apparatus for a gas display panel and a method to provide reliable write, sustain and erase operations. For sustain operation, a first square wave train is applied to all horizontal lines of the gas display panel simultaneously as a second square wave train displaced 90.degree. from the first square wave train is applied to all vertical lines. For a write operation, the frequency of the first and second square wave trains is reduced and a pulse is superimposed or algebraically added to the sustain signals to provide a composite signal having an amplitude and duration sufficient to ionize the gas at a selected intersection. The superimposed signal increases the potential on a selected horizontal line, decreases the potential on the remaining horizontal lines, decreases the potential on a selected vertical line and increases the potential on the remaining vertical lines. The selected cell thus receives an increased potential difference sufficient to equal or exceed the firing potential after all the remaining cells receive a sustain potential which reignites all cells which were previously ignited. The algebraically added pulses cancel out the effect on each other across the half-select cells and the nonselected cells. For an erase operation a given signal of constant magnitude and polarity is applied to all horizontal lines and all vertical lines and a pulse is algebraically added on the given line which (a) increases the potential on a selected horizontal line, (b) decreases the potential on the nonselected horizontal lines, (c) decreases the potential on a selected vertical line, and (d) increases the potential on the nonselected vertical lines whereby no gas cell or site in the gas panel receives a potential difference sufficient to equal or exceed the sustain level. However, the selected gas cell, and only this gas cell, receives a potential difference having a polarity opposite to that of the last sustain signal and an amplitude that is just barely sufficient to fire the cell, and this is effective in reducing the wall charge across the selected gas cell substantially to zero. After a suitable time delay, referred to as dead time, the wall charge across the selected cell is reduced to zero, and the selected gas cell is thus returned to the extinguished state. A sustain operation then takes place which reignites all gas cells previously ignited before the erase operation except the selected erased cell. The algebraically added pulses cancel out the effect of each other across the half-selected cells and the nonselected cells.
Accordingly, it is a primary object of the present invention to provide improved interface and logic circuit arrangements for use with a gaseous discharge display/memory device.
It is a further feature of this invention to provide an improved gaseous discharge display system of high reliability having low voltage and power requirements for the logic and the panel driving circuitry and adapted for the use of integrated circuitry.
Another object of the present invention is to provide improved low voltage logic selection circuitry for a gaseous discharge device adapted to communicate with a low signal level processor and a high signal level sustain generator.
Still another object of the invention is to provide an improved gaseous discharge display control system in which low level logic and write pulsing circuitry are floated on the sustainer voltage of the gaseous discharge display device to permit integrated circuit packaging of the logic and panel driving circuits.